You can import images to make floors as of IFFSnooper version 1.1.3. Use Import -- Floor Tile or Cmd-Fto choose an image file which you can then convert and save as a .flr file. The base image for a floor should be 128 x 128 pixels.
Example of image ready to transform into a floor tile (original Maxis artwork).
If your image is too big (as in this case) it will be cropped to fit.
The transformed image is now ready to save as a floor tile. The yellow background will not appear in the floor tile sprites.
Once you have seen what the sprites will look like, you can encode them and save the floor file using the File -- Save menu command or the Save button.
The finished floor tile in game.
Making Floor Tiles